1. Field of the Invention
This invention relates to a lithium ion secondary battery comprising a positive electrode and a negative electrode facing each other via a separator supporting an electrolytic solution. More particularly, it relates to a battery structure providing improved electrical connections between each of a positive electrode and a negative electrode and a separator so that a battery may have an arbitrary shape, such as a thin shape, and a process for forming such a structure.
2. Discussion of the Background
There has been an eager demand for reduction in size and weight of portable electronic equipment, and the realization relies heavily on improvement of battery performance. To meet the demand, development and improvement of a variety of batteries have been proceeding. Battery characteristics expected to be improved include increases in voltage, energy density, resistance to high load, freedom of shape, and safety. Of available batteries, lithium ion batteries are secondary batteries that have achieved the most a high voltage, a high energy density, and excellent resistance to high load and will undergo successive improvements.
A lithium ion secondary battery mainly comprises a positive electrode, a negative electrode, and an ion conducting layer interposed between the electrodes. The lithium ion secondary batteries that have been put to practical use employ a positive electrode plate prepared by applying to an aluminum current collector a mixture comprising a powdered active material, such as a lithium-cobalt complex oxide, a powdered electron conductor, and a binder resin; a negative electrode plate prepared by applying to a copper current collector a mixture of a powdered carbonaceous active material and a binder resin; and an ion conducting layer made of a porous film of polyethylene, polypropylene, etc. filled with a nonaqueous solvent containing lithium ions.
FIG. 9 schematically illustrates a cross section of a conventional cylindrical lithium ion secondary battery disclosed in JP-A-8-83608. In FIG. 9 reference numeral 1 indicates a battery case made of stainless steel, etc. which also serves as a negative electrode terminal, and numeral 2 an electrode body put into the battery case 1. The electrode body 2 is composed of a positive electrode 3, a separator 4, and a negative electrode 5 in a rolled-up form. In order for the electrode body 2 to maintain electrical connections among the positive electrode 3, the separator 4, and the negative electrode 5, it is necessary to apply pressure thereto from outside. For this purpose, the electrode body 2 is put into a firm metal-made case to maintain all the planar contacts. In the case of rectangular batteries, an external pressing force is imposed to a bundle of strip electrodes by, for example, putting the bundle in a rectangular metal case.
That is, a contact between a positive electrode and a negative electrode in commercially available lithium ion secondary batteries has been made by using a firm case made of metal, etc. Without such a case, the electrodes would be separated, and the battery characteristics would be deteriorated due to difficulty in maintaining electrical connection between electrodes via an ion conducting layer (separator). However, occupying a large proportion in the total weight and volume of a battery, the case causes reduction in energy density of the battery, Moreover, being rigid, it imposes limitation on battery shape, making it difficult to make a battery of arbitrary shape.
Under such circumstances, development of lithium ion secondary batteries which do not require a case has been proceeding, aiming at reductions in weight and thickness. The key to development of batteries requiring no case is how to maintain an electrical connection between each of a positive electrode and a negative electrode and an ion conducting layer (i.e., separator) interposed therebetween without adding an outer force. Connecting means requiring no outer force that have been proposed to date include a structure in which electrodes and a separator are brought into intimate contact by means of a resin and the like.
For example, JP-A-5-159802 teaches a method in which an ion conducting solid electrolyte layer, a positive electrode, and a negative electrode are heat-bonded into an integral body by use of a thermoplastic resin binder. According to this technique, electrodes are brought into intimate contact by uniting the electrodes and an electrolyte layer into an integral body so that the electrical connection between electrodes is maintained to perform the function as a battery without applying outer force.
As mentioned above, conventional lithium ion secondary batteries having the above-mentioned structures have their several problems. That is, those in which a firm case is used for ensuring intimate contacts between electrodes and a separator and electrical connections between electrodes have the problem that the case which does not participate in electricity generation has a large proportion in the total volume or weight of a battery, which is disadvantageous for production of batteries having ahigh energy density. Where the proposed method comprising bonding electrodes and an ion conductor with an adhesive resin is followed, for example, where a solid electrolyte and electrodes are merely brought into contact via an adhesive resin, the resistance to ion conduction within a battery increases due to the great resistance of the adhesive resin layer, resulting in reductions of battery characteristics.
Further, the battery according to JP-A-5-159802 supra, in which electrodes and a solid electrolyte are joined with a binder, is disadvantageous in terms of ion conductivity as compared with, for example, batteries using a liquid electrolyte because the interface between an electrode and an electrolyte is covered with the binder. Even though an ion-conducting binder is employed, there is no binder generally known to be equal or superior in ion conductivity to a liquid electrolyte, and it has been difficult to achieve battery performance equal to that of a battery using a liquid electrolyte.